Choosing a mattress that is of great quality and is right for your requirements is a  complex matter. When in doubt, select a futon mattress, as they are the best of options available.

The mattresses are available in different forms and types. Of course you will have to make some effort in selecting the right one.

A futon mattress should be available  in a frame with at least a five-year warranty and should be able to serve as both a bed and a couch.
 
Frames are of two types. They are the trifold and bifold frame.  The former is folded twice and uses the width of the mattress as the couch, whereas the latter is folded once, provides more seat room and uses the length of the mattress as the couch.
       
A futon mattress should be at least 6 inches thick and made up either of cotton, or a combination of cotton-foam, cotton wool, and cotton-polyester.  The mattress should be heavy in terms of weight. This makes the mattress durable.

A weightless futon is made of cotton and foam but does not sag. As compared with lighter mattress, futon made of cotton and polyester is very flexible and soft.

If you prefer  super soft material then futon mattress made of cotton and wool mixture is just right for you. Generally wool mattresses are not flexible and should be tested before use. You can test them by lying down on them and checking how comfortable and supportive they are. They should not be lumpy anywhere.

The frames used in futon mattresses should of the correct size and made preferably from southern yellow pine, which is heavy and durable. A perfect futon cover always protects the mattress from dust and stains.